The human body cannot identify toxins i.e. the human body cannot think. The human body can only react to exogenous stimulus. It's such a common misconception that our bodies can actively find and know noxious stimuli. It's likely because growing up, we likely all had parents who told us vomiting or diarrhea was our body's way of getting rid of the bad stuff. However, that is just not the case...it's comforting to hear that as a child, but it's actually quite the opposite in reality. Reality is that bacteria and viruses have evolved proteins that bind to and activate certain receptors in our gastrointestinal tract and cause a multitude of different effects. For instance, in the majority of food poisoning cases, the toxin is likely an emetic toxin that literally binds to and activates the 5HT-3 receptor in the small intestine. This is a serotonin receptor that directly activates the vomiting center in the brain, so when you consume a food that has been improperly stored/contaminated by a bacterium that produces an emetic toxin, that toxin directly activates your vomiting center. These toxins and the genes that encode them have been highly conserved over time as they allow the bacteria that make them to spread rapidly through the environment. That's really all a bacteria or virus wants to do....multiply and spread...but please don't take that as me saying that bacteria and viruses are thinking. They are not...haha.